Relative velocity in Mean Free Path problems

[casanova2528]

Why is average relative velocity equal to the square root of 2 times the magnitude of the x direction velocity?

I'm trying to solve some problems that deal with gas laws from Ideal gases to the Van der Waals gases.

Does anybody have any ideas?

Thanks.

 

[Oojee]

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/menfre.html#c5